Flexible offshore pipes comprising a tube-formed inner liner and at least one reinforcement layer are generally used for the transportation of oil and gas products over long distances and often at elevated temperatures, such as above 60° C. or more.
Offshore pipes are also used for injection of chemicals into a sub-sea drilled well e.g. connected between a host oil platform and a sub-sea satellite installation.
Offshore pipes must be capable of operating at high pressures, and the pipes should be resistant to chemicals and water, including seawater. Furthermore such offshore pipes should be flexible so that they can be spooled onto a drum or reel.
Offshore pipes are normally very long, so-called risers often several hundred meters long and so-called flow-lines often several kilometers long. They are laid on the seabed, typically subjected to high pressures and pressure differences along the pipeline. When the pipeline is transporting oil or gas, the pipelines may be exposed to temperatures substantially above 60° C. The offshore pipes should therefore be capable of operating at high temperatures and high pressures.
Offshore pipes generally comprise one or more tube-formed barrier layers including an inner liner and at least one reinforcing layer. The inner liner is the innermost polymer layer, which in known offshore pipes also constitutes a barrier layer, and which is exposed to a fluid, e.g. oil transported in the pipeline. In most situations, the pipeline also comprises an outer sheath providing a barrier to the outer environment such as seawater. The pipe normally comprises one or more reinforcing layers between the inner liner and the outer sheath, and some pipes also comprise a reinforcing layer inside the pipe, called a carcass. The carcass prevents collapse of the inner liner and provides mechanical protection to the inner liner. Some pipes also comprise one or more intermediate polymer layers.
The inner liner should be chemically stable and mechanically strong even when subjected to high temperatures. Furthermore, the inner liner should be manufactured in one piece since repair, welding or other types of connecting methods are not acceptable for inner liners in offshore pipelines. The inner liner is therefore normally produced by continuous extrusion of a polymer. A number of polymers are presently used for the production of inner liners, such as Polyamide-11 (PA-11), polyethylene (PE) and Polyvinylidene diflouride (PVDF).
These materials shall fulfill the combined requirements of e.g. heat stability, resistance to crude oil, seawater, gases, mechanical fatigue, ductility, strength, durability and processability. The inner liner material is normally selected on a case-to-case basis after careful investigation of the conditions for the planned installation. Here, cross-linked polyethylene may in many cases prove to fulfill the requirements.
Additionally, the interest in use of inner liners in corrosive applications with high concentrations of carbon dioxide and/or hydrogen sulphides is increasing. Furthermore, polyamides are susceptible to hydrolysis and aliphatic polyketones are also susceptible to degradation at elevated temperatures. However, the permeability of gases increases with temperature, and polyethylene has a relatively high permeability to gases. Thus permeation of gases like methane, carbon dioxide and hydrogen sulphide may in some cases be prohibitive for use of cross-linked polyethylene inner liners at high temperatures.
In EP 487 691 it has been suggested to use an inner liner of cross-linked polyethylene. An inner liner with such cross-linked material has shown to be highly improved compared to inner liners of the similar non-cross-linked (thermoplastic) material.
In order not to degrade the material, the process in the prior art of producing an inner liner is carried out in two steps, first the material in non-cross-linked form is manufactured by extrusion, and afterwards the material is cross-linked. When the material is cross-linked, it is only to some degree possible to change its shape without degrading the material.
The cross-linking step is often very cumbersome and time and space demanding. EP 487.691 describes the cross-linking step of silanized polyethylene with reference to the figures. The pipeline is first manufactured by extrusion of the inner layer of polyethylene, followed by metal armouring and outer sheathing. The entire multilayer pipe structure is mounted with end fittings, and the flexible pipe is connected to a device for circulating water in the inner liner tube. The water is heated to about 92-98° C. and circulated using pumps. The time of cross-linking is between 48 and 120 hours followed by a cooling step for about 20 hours.
By this process, it is necessary to manufacture the entire pipe before making the actual cross-linking of the inner liner. In case of a quality problem of the inner liner, it appears impractical to make the entire pipe without assuring final properties of the cross-linked inner liner. The patent describes both the use of a Sioplas® process involving peroxide-activated grafting of the vinylsilane onto the polyethylene in a separate compounding step, and the Monosil® process with in-situ silane grafting of polyethylene. It is preferred to use a dibutyltindilaurate (DBTDL) as cross-linking accelerator and a density of the polyethylene above 931 kg/m3, preferably over 940 kg/m3.
The required properties for the other polymer layers, intermediate layer(s) and outer layer are much similar to the required properties of the inner liner.
A number of methods of producing PE based covers for cables are known e.g. as disclosed in U.S. Pat. Nos. 4,528,155 and 3,868,436, by extrusion of PE comprising a heat activatable peroxide, followed by subjecting the cover to heat e.g. by steam or pressurized Nitrogen to thereby initiate the cross-linking thereof.